Dental prosthesis

ABSTRACT

A dental composition comprising self-lubricating particles and formed into a dental tooth. The invention provides a dental tooth formed from a composition which includes polymeric matrix material and particles of ultrahigh molecular weight polyethylene having a particle size less than 500 microns, and a weight average molecular weight greater than 1,000,000 g/mole chemically bonded to the matrix material and the composition formed into an aesthetic dental tooth. The tooth has an outer face, and at least one of the particles is smeared over the outer face.

This application is a continuation of patent application Ser. No.09/333,727 filed Jun. 15, 1999, now U.S. Pat. No. 6,139,322, which is acontinuation of patent application Ser. No. 08/447,296, filed May 22,1995; now abandoned, which is a division of patent application Ser. No.07/979,093 filed Nov. 19, 1992 now abandoned, which is acontinuation-in-part of patent application Ser. No. 07/729,018 filedJul. 12, 1991 now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to self-lubricating abrasion resistantcompositions useful for a wide range of applications. Particular utilityis found in the dental and medical arts where such compositions aresuitable for the formation and construction of dental prosthesis such asartificial teeth, inlays, onlays, and facings, crowns and bridges andartificial bone parts and medical prosthetic body parts, such as kneejoints and/or other bone engaged surfaces and the like where abrasionresistance, polishability and: aesthetics are important.

Artificial teeth should exhibit certain physical and physiochemicalcharacteristics to be suitable for use. They should be hard andresistant to chipping, durable, and stable to solvents, water, and heat.In addition, they should be of an aesthetically acceptable color, i.e.,close to that of natural teeth, or be amenable to artificial coloration.The teeth should not cause excessive wear to opposing natural orartificial teeth, should not wear out of occlusion, and should becapable of being bonded firmly to supportive structures. They shouldalso be amenable to ordinary means of physical shaping, grinding, andpolishing, so as to minimize production costs.

Various metals and ceramics as used in the formation of prior artartificial teeth and other dental appliances possess certain inherentdeficiencies which lessen their desirability in dental applications.Thus, the metallic color of gold, amalgam, and other metallic speciesserves as an aesthetic detraction to the wearer of appliances madetherefrom. In addition, the high cost of most noble metals from whichmany such appliances are commonly made leads to a cost considerationwhenever their use is contemplated. Ceramic materials, another commonalternative, are often difficult to form into acceptable shapes, and maytend to be abrasive and aesthetically unpleasant subsurfaces upon thephysical wearing of surface layers. Such materials are also difficult topolish satisfactorily. These reasons together with factors related tocost, to consumer preference, to the technical skills of dentalpractitioners, and to convenience have motivated a search foralternative compositions suitable for the construction of dentalappliances, inlays, onlays, crown and bridge material, artificial teethand the like.

Of the presently available organic compositions used for theconstruction of artificial teeth, most are composed of acrylics, oftencrosslinked by polyfunctional moieties.

As used herein “self-lubricating material” means a material which isadapted to increase the lubricity of a product surface. Preferablyself-lubricating material for use as particles in prostheses inaccordance with the invention has a kinetic coefficient of friction lessthan 0.25 when measured using ASTM D 1894-78 with a load of 5 psi.

As used herein ASTM D 1894-78 is carried out using water and the samematerials.

As used herein “water insensitive” means that water does not have amaterial effect upon the material so characterized.

It is to be understood that the term “bisphenol-A” is commonly utilizedin the art to indicate the chemical compound2,2-bis(4-hydroxyphenyl)propane. It is also to be understood that theterm “bis-GMA” is commonly used to indicate the chemical compound2,2-bis(4-(2-hydroxy-3-methacryloxypropoxy)phenyl)propane, otherwisereferred to as “digycidyl methacrylate ester of bisphenol-A.”

Dentsply U.S. Pat. Nos. 4,396,476, 4,396,377 and 4,698,373 (thedisclosures of which are incorporated herein by reference) discloseinterpenetrating network teeth but do not disclose self-lubricatingabrasion resistant compositions as required by the applicant'sinvention.

Thornton U.S. Pat. No. 2,345,305 discloses making artificial teethcomprised of different plastic materials for the face (“enamel”) andbody portions. Note FIG. 17, and page 4, column 2, lines 21-24. Anothercomposite plastic tooth structure is disclosed by Cornell U.S. Pat. No.3,488,846.

The Rosenkranz et al. U.S. Pat. No. 3,928,299 discloses an organichomopolymer or random copolymer containing urethane groups.

Michl et al. in U.S. Pat. Nos. 4,267,097 and 4,281,991 (the disclosuresof which are incorporated herein by reference) disclose artificial teethprepared from (a) particle/bead PMMA, (b) a liquid monomer such as theadduct of hydroxymethacrylates and diisocyanates or difunctional estersof (meth)acrylic acids or mixtures thereof, and (c) micro-fine inorganicfillers. Michl et al do not disclose self-lubricating abrasion resistanthardenable compositions as required by Applicants' invention.

Walkowiak et al. in U.S. Pat. Nos. 4,308,190 and 4,369,262 disclosedental paste materials of a polymerizable acrylic ester, a crosslinkedbead polymer, and a particulate inorganic filler and do not discloseself-lubricating abrasion resistant hardenable compositions, orinterpenetrating network compositions for making artificial teeth asrequired by Applicants' invention.

Simpson in U.S. Pat. No. 4,361,676 discloses a sag-resistant, pumpablecomposition comprising a liquid material dispersed throughout asynthetic, continuous crosslinked polymer matrix.

Wright et al. in U.S. Pat. No. 4,598,111 disclose the use of variousdivinyl compounds, including divinyl dimethylsilane (column 6, line 35)as a crosslinking agent for (meth)-acrylate monomer systems. Otherpatents of this general type include, for example, Kohno et al. U.S.Pat. No. 4,761,436; dimethyldivinylsilane as a co-monomer; column 3,line 29); Feinberg et al. U.S. Pat. No. 4,894,315; column 3, lines37-38); Fryd et al. U.S. Pat. No. 4,956,252; column 5, lines 43-44); andKafka et al. U.S. Pat. No. 4,970,037; column 9, lines 16-17).

Yamazaki et al. in U.S. Pat. No. 4,826,893 disclose a dental compositioncomprising (a) a siloxane polymer, (b) a monomer copolymerizable withthe siloxane polymer, (c) a polymerization catalyst, e.g. benzoylperoxide, and optionally, (d) a filler.

Laundry in U.S. Pat. No. 3,084,436 discloses soft dental materialsmanufactured from mixtures of methacrylate monomers. Monofunctionalesters together with vinyl-acetate or vinyl stearate are crosslinkedwith polyfunctional esters of acrylic or met acrylic acid. The resultingproduct is disclosed as being three dimensionally crosslinked.

Graham et al. in U.S. Pat. No. 3,087,875 disclose preparation of graftcopolymers. Alkyl methacrylate and analogous polymers are dissolved inmonomers such as alkyl acrylates, alkyl thioacrylates, and N-vinyllactams. The monomers are subsequently grafted to the preformed polymersvia photochemical initiation.

Cornell in U.S. Pat. No. 3,427,274 discloses hard-enable materialsformed from a mixture of methyl methacrylate homopolymer andstyrene-butadiene copolymer latex coated with methyl methacrylatepolymer which may be incorporated in a methacrylate-crosslinking agentcomposition to form hardenable compositions.

Chang in U.S. Pat. No. 3,452,437 discloses a dental restorative materialformed from the “diglycidyl methacrylate of bisphenol-A” (bis-GMA) towhich a quantity of methyl methacrylate may be added.

Bruckmann et al. in U.S. Pat. No. 3,468,977 disclose the formulation ofdental compositions from a mixture of a polymer and a monomer. Thepreformed uncrosslinked polymer beads are allowed to swell with monomerwhich may contain a crosslinking agent. Acrylic materials may be usedfor both the monomer and the polymer. Petner in U.S. Pat. No. 3,470,615,teaches the formulation of a material suitable for use in theconstruction of dental appliances. A mixture of an uncrosslinkedhomopolymer and crosslinked copolymer is dissolved in a liquidpolyglycol dimethacrylate to form a suspension which may be brushed on asubstratum and subsequently hardened by heat to build up layers ofpolymeric material. A similar teaching may be found in U.S. Pat. No.3,471,596, also to Petner et al. A thick liquid is provided which isuseful in the building up of dental crowns and the like. Thedifunctional monomer may contain various thickening agents includingpoly(methyl methacrylate). In some embodiments, the poly (methylmethacrylate) may be supplemented with additional polymer which may bepartially crosslinked with allyl methacrylate.

Lee in U.S. Pat. No. 3,539,533 discloses a filling material including amonomer solution filled with inorganic particulate filler. The monomersolution may be a mixture of methacrylate monomers containingbisphenol-A dimethacrylate.

Taylor in U.S. Pat. No. 3,597,389 discloses polyfunctional methacrylatemonomers, including “bis-phenol-A glycidyl dimethacrylate” (bis),polymerized with an inorganic filler to yield dental compositions.

Waller in U.S. Pat. No. 3,629,187 discloses the use of the isocyanate ordiisocyanate adducts of bisphenol-A type compounds. These adducts areemployed together with various inorganic fillers and liquid monomers toform liquid or paste compositions which are polymerizable eitherthermally or photochemically.

Dougherty in U.S. Pat. No. 3,647,498 discloses dental compositions whichare composed of liquid-solid mixtures. The solid phase is an acrylate ormethacrylate polymer in bead form.

Logemann in U.S. Pat. No. 3,649,608 discloses dental compositions whichcomprise solid bead polymers or copolymers of methacrylate typematerials.

Lee in U.S. Pat. No. 3,751,399 discloses compositions for dental usecomprising aromatic and alicyclic polyacrylates which are mixed togetherwith other polyacrylate compounds especially those containingbisphenol-A structures.

Sperling in U.S. Pat. No. 3,833,404 discloses elastomers, especiallyacrylates, urethanes, butadienes, natural rubbers, and polyvinylalcohol, are formulated which possess interpenetrating polymeric networktype structures. These materials are disclosed as being “hard”, but areused as vibration and sound damping insulators.

Highgate in U.S. Pat. No. 3,961,379 discloses an article manufacturedfrom a crosslinked polymer which is swollen with a monomer containing acrosslinking agent.

Temin in U.S. Pat. No. 4,197,234 discloses dental restorative compositecompositions and filler therefor.

Engel in U.S. Pat. No. 4,288,221 discloses durable polishable directfilling material.

Jarby in U.S. Pat. No. 3,469,317 discloses material for fillingcavities.

Crowell in U.S. Pat. No. 2,315,503 discloses art of molding compositeresins.

Crowell in U.S. Pat. No. 2,403,172 discloses art of molding resins ofvinyl type.

Van Beuren Joy in U.S. Pat. No. 3,532,502 discloses dentures,compositions, and methods.

Michl et al in U.S. Pat. No. 4,281,991 discloses dental prostheses.

Bauman et al in U.S. Pat. No. 4,771,110 discloses polymeric materialshaving controlled physical properties and processes for obtaining these.

Muramoto et al in U.S. Pat. No. 4,829,127 discloses composite resinparticles, its preparation and resinous composition for coating usecontaining the same.

Bauman in U.S. Pat. No. 4,880,879 discloses abrasion resistant compositematerial and process for making the same.

Podszun et al in U.S. Pat. No. 4,937,144 discloses dental fillers.

Lee in Australian Patent Specification 50,674 discloses dental adhesivecomposites.

None of the foregoing patents discloses the novel compositions andprostheses having matrix material bonded or adapted to be banded toself-lubricating abrasion resistant particles in accordance with theinvention.

OBJECTS OF THE INVENTION

It is an object of the invention to provide abrasion resistant dentalcompositions especially useful as dental appliances, dentures and otherprosthesis, inlays, onlays, facings, crowns and bridges and the like.

It is an object of the invention to provide a tooth which includesself-lubricating polymer.

It is an object of the invention to provide a dental compositionincluding a polymeric matrix material and self-lubricating particles.

It is an object of the invention to provide a composition for dentaluses, such as the formation of prosthetic devices, includingself-lubricating material, a monomeric matrix forming material andinorganic filler.

It is an object of the invention to provide a dental compositionincluding self-lubricating particles.

It is an object of the invention to provide a dental composition havingself-lubricating abrasion resistant particles which are supported by andchemically bonded to a polymeric matrix material.

It is an object of the invention to provide a dental product, such as, adental prosthesis, artificial tooth, inlay, onlay, facing, crown orbridge which is wear resistant and self-lubricating across its entirecross section.

It is an object of the invention to provide an artificial toothincluding polyethylene particles having a molecular weight of at least500,000, and a particle size less than 80 microns and are supported byand bonded to in an interpenetrating network polymer composition.

It is an object of the invention to provide compositions which areuseful in the construction of artificial teeth and other dentalappliances, which compositions lead to products having superior physicaland aesthetic characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of an posterior tooth inaccordance with the invention.

FIG. 2 is a front view of an posterior tooth in accordance with theinvention.

BRIEF SUMMARY OF THE INVENTION

Dental composition and prosthesis including a self-lubricating material.The dental composition is used to form dental products having reducedwear. The dental products formed are abrasion resistant andself-lubricating across their entire cross sections. In a preferredembodiment the self-lubricating material is selected from among thosehaving coefficients of friction one-half or less of that for polymethylmethacrylate. The particles are reacted so they may be chemically bondedto the polymer matrix within which they are dispersed. In a preferredembodiment polyethylene particles having a molecular weight of at least1,000,000, and a particle size less than 80 microns are dispersed withinan acrylate or methacrylate polymer matrix. The composition is formedinto a dental prosthesis, such as an artificial tooth, inlay, onlay,facing, crown or bridge.

DETAILED DESCRIPTION OF THE INVENTION

Dental compositions of matter are described which includeself-lubricating polymeric material. The dental compositions are moldedto form dental prosthesis having reduced abrasive wear. While notwishing to be held to any theory describing the mechanism operative bywhich additions of self-lubricating polymer improves abrasionresistance, it is hypothesized that as the matrix polymer is worn awaynew particles of the self-lubricating abrasion resistant polymer bondedto the matrix are exposed which smear over the surface of the moldedarticle to provide a modified surface with lower coefficient of frictionand consequent improved abrasive wear resistance. Self-lubricatingparticles are bonded to the martix material to avoid being torn orotherwise removed in-toto from the matrix during conditions of abrasivewear. The dental produces formed show improved abrasion resistant andpreferably are self-lubricating across their entire cross sections. In apreferred embodiment the self-lubricating material preferably includespolyethylene particles having a molecular weight of at least 1,000,000,and a particle size less than 80 microns. The composition is formed intoa dental prosthesis, artificial tooth, inlay, onlay, facing, crown orbridge.

The invention is now discussed with more particular reference to FIGS. 1and 2 in which like numerals refer to the same component. FIGS. 1 and 2show an artificial posterior tooth 30 having a tooth body 32 and anenamel coating 34. Tooth 30 includes ridge lap 36, buckle face 38,occlusal face 40 and lingual face 42. In a preferred embodiment toothbody 32 includes high molecular weight self-lubricating particles in aninterpenetrating polymer network (IPN) composition. Enamel 34 includesself-lubricating abrasion resistant particles in an interpenetratingnetwork composition.

The self-lubricating material preferably is added in the form ofparticles and fibers. Most preferably, these self-lubricating particlesare micron-sized particles of ultra-high molecular weight polyethylenethat have been functionalized by oxidation by gas plasma treatmentpreferably in the presence of a fluorine containing gas. Preferablyparticles of high molecular weight polyethylene, included asself-lubricating material in dental prosthesis in accordance with theinvention, have a particle size less than 80 microns.

Self-Lubricating

Preferably the self-lubricating abrasion resistant material includes apolymers and copolymers of the general formula (I):

wherein n is an integer from 100 to 1,000,000,(--) is a single or a double bond;when (--) is a double bond R, R′, and R₃ independently are hydrogen, ora lower alkyl of from 1 to 6 carbons, and R₁ and R₂ are not present;when (--) is a single bond R, R′, R₁, R₂, and R₃ independently arehydrogen, fluorine or a lower alkyl of from 1 to 6 carbons.

Such polymers have a coefficient of friction of less than about one-halfthat of polymethyl methacrylate, or about 0.25 or less when measuredusing ASTM D 1894-78 with a load approximating 5 psi. An essentialcharacteristic of the polymers and copolymers chosen is that theirsurface is capable of being bonded to the chosen matrix. A preferredmethod of achieving this is to oxidize or otherwise functionalize theparticle surface as, for example, through oxyfluorination at elevatedtemperatures or gas plasma treatment. This requires the abstraction ofhydrogen from the polymer chain. Accordingly polytetrafluoroethylene orpolytetrachloroethylene are unsuitable for use according to thisinvention. In contradistinction polymers such as trifluoroethylene ordifluoroethylene, monofluoroethylene or polyethylene or polyproppylene,for example, with molecular weights greater than 500,000 or preferably1,000,000 or more which may be functionalized in this manner are amongpreferred self-lubricating materials.

In a preferred embodiment ultrahigh molecular weight polyethylene withdynamic coefficient of friction in water against steel of 0.05-0.10 wasutilized as the self-lubricating material (“Ultrahigh Molecular WeightPolyethylenes” by H. L. Stein,: Engineering Plastics, Vol.2, 1988, ASM,Materials Part, Ohio). In comparision, the static coefficient offriction of polymethylmethacrylate against steel (CRC Handbook ofChemistry and Engineering, 60th edition, 1190, pages F-21, F-23) is0.4-0.5.

Preferably self-lubricating material within the scope of general formula(I) is in the form of particles which have been treated with a fluorinecontaining gas as disclosed by Bauman et al in U.S. Pat. No. 4,771,110,incorporated herein by reference in its entirety to generage functionalreactive sites in the surface layer of the particles. For example,particles of UHMWPE having functional reactive sites in the surfacelayer are sold by Air Products and Chemicals, Inc. under the trademarkPRIMAX.

Dental Uses

Artificial teeth and other dental prostheses which are prepared fromhardenable dental compositions in accordance with one preferredembodiment of the invention have outer face(s) which include polymericself-lubricating material. For example, occlusal portions, i.e.“enamel”, of artificial teeth are molded from compositions of theinvention and are laminated over the tooth body (32 of FIG. 1) which maybe made from less abrasion resistant prior art materials. Alternatively,enamel and body portion may be molded from the material of theinvention.

In general, the novel compositions of this invention are useful for theformation, construction and repair of dental appliances, artificialteeth, oral prosthesis, and similar articles. In addition, compositionsin accordance with the invention are utilized especially as inlay oronlays cemented into or onto teeth, and in the preparation of dentalcrowns and bridges.

The hardenable dental molding compositions of a preferredinterpenetrating network embodiment of the invention include blends ofpowder and liquid components which are combined in certain proportionsand permitted to age or mature to produce a precursor blend that ismoldable into prosthetic teeth and other dental devices. The precursorblend is formed by combining polymer and monomer which is thenpolymerized.

The hardenable dental molding compositions of the invention comprise ablend of powder and liquid components which are combined in certainproportions and are permitted to reach a moldable consistency and arethen molded and polymerized by heat and light into a useful desiredform. The resulting form may be the finished dental device or it may bemachined or otherwise subsequently post-formed to produce the desiredshape, as for example a dental inlay formed from a computer assisteddesign and machining device. The powder may, for example, be comprisedof a self-lubricating material, an acrylate or methacrylate polymer orcopolymer, a monomer, a crosslinking monomer and/or oligomer, apolymerization catalyst, and optional inorganic filler such as silica,glass, or ceramic.

In a preferred embodiment of the invention product compositions includeself-lubricating particles dispersed within an interpenetrating polymernetwork. Product compositions are formed from precursor blends.Precursor blends are formed by combining particulate self-lubricatingmaterial, a crosslinked polymer, a monofunctional monomer and/or acrosslinking monomer and/or oligomer. Optionally precursor blendsinclude uncrosslinked polymer and a polymerization catalyst. Precursorblends are allowed to age or mature and then are molded and polymerized.

The crosslinked polymer is in the form of discrete particles havingaverage diameters ranging form about 0.001 micron to about 500 microns.Preferably, at least 50% by weight of the particles have diameters lessthan about 150 microns, and more preferably, less than 100 microns. Amixture of two or more different crosslinked polymers may be used. Acharacteristic of the crosslinked polymer is that it will be insolublein, but will be swollen by the liquid components used in the preparationof the precursor blend. Uncrosslinked polymer may be characterized asbeing capable of dissolving in or being dispersed by the liquidcomponents of the blend. The liquid polymerizable monomer blendcomponent of the compositions of the invention have the capacity todissolve or disperse such uncrosslinked polymer, dissolve or becomemiscible with the crosslinking agent and swell the particles ofcrosslinked polymer. A mixture of two or more polymerizable mono andpolyfunctional polymerizable monomers which dissolve or become misciblewith the crosslinking agent and swell the particles of crosslinkedpolymer are used as the polymerizable monomer component of thecomposition of the invention.

Crosslinked powder used in a preferred embodiment of the invention isprepared from polyfunctional and/or monofunctional monomers which arepolymerized, e.g. by bulk, solution, suspension or emulsion technics andcomminuted to the preferred size ranges if necessary.

Monofunctional Monomers for Preparation of Crosslinked Polymer Powder

Monomers useful in the production of the crosslinked polymers used inthe practice of preferred embodiments of the invention include methyl-,ethyl-, isopropyl-, tert-butyloctyl-, dodecyl-, cyclohexyl-,chloromethyl-, tetrachloroethyl-, perfluorooctyl-, hydroxyethyl-,hydroxypropyl-, hydroxybutyl-, 3-hydroxy-phenyl-, 4-hydroxy-phenyl-,aminoethyl-, aminophenyl-, and thiophenyl-, acrylate, methacrylate,ethacrylate, propacrylate, butyl acrylate and chloromethacrylate, aswell as the homologous mono-acrylic acid esters of bisphenol-A,dihydroxydiphenyl sulfone, dihydroxydiphenyl ether, dihydroxybiphenyl,dihydroxydiphenyl sulfoxide, and 2,2bis(4-hydroxy-2,3,5,6-tetrafluorophenyl)propane, as examples only. Othersuitable species will be apparent to those skilled in the art, some ofwhich are later recited below.

Polyfunctional Monomers for Preparation Of Crosslinked Polymer Powder

Crosslinking agents which are useful in the production or thecrosslinked polymer component of a preferred embodiment of the inventioninclude a wide variety of di- or polyfunctional moieties which arecapable of crosslinking monomer species, for example, acrylic and loweralkyl acrylic acid diesters, acrylic and lower alkyl acrylic acid estersformed from alcohols having a second reactive function such as allylmethacrylate, urethane diacrylates and dimethacrylates, polyvinyliccompounds, divinyl aromatic compounds, esters of di- or polyfunctionalunsaturated acids, e.g., maleic, fumaric, citraconic, mesaconic,itaconic, malonic, or aconitic, etc., acids preferably reacted witheither monohydric or poly-hydroxylic saturated and unsaturated alcoholsto form esters which are effective polyfunctional crosslinking agentsuseful in the formulation of the crosslinked polymers of the invention.In general, these alcohols have one or more hydroxylic functionalitiesand have from 2 to about 30 carbon atoms. Thus, useful alcohols includeallyl, methallyl, crotyl, vinyl, butenyl, isobutenyl and similarunsaturated alcohols as well as polyols such as ethylene glycol,propylene glycol, butylene glycol, diethylene glycol, triethyleneglycol, tetraethylene glycol, penta-ethylene glycol, glycerol,1,3,3-tri-methylolpropane, pentaerythritol, dihydroxyphenol, andalkylidene bis-phenols such as bisphenol-A,1,1-bis(4-hydroxy-phenyl)-methane, 4,4′dihydroxybiphenyl,4,4′-dihydroxydiphenyl sulfone, dihydroxydiphenyl ether,dihydroxydiphenyl sulfoxide, resorcinol, hydroquinone, etc. and estersof a mono- or dibasic unsaturated acid with an unsaturatedmono-hydroxylic alcohol such as allyl acrylate, allyl methacrylate,vinyl acrylate (methacrylate and C₁ to C₂₀ homologs), dimethallylfumarate, N-allyl acrylamide, crotyl acrylate, allyl crotonate, allylcinnamate, diallyl maleate, etc. di-, tri-, and higher esters ofpolyhydroxylic alcohols such as ethylene “glycol” diacrylate(dimethacrylate and C₂-C₄₀ homologs), trimethylolpropanetrimethacrylate, the diacrylate and dimethacrylate esters ofbisphenol-A, as well as acrylate and alkyl acrylate esters whichcorrespond to the general formula (II):

where R and R, may be the same or different and are hydrogen or alkylgroups containing from 1 to about 6 carbon atoms and n is a whole numberfrom 1 to about 10. Alternatively, the crosslinking agent may conform tothe formula (III)

where R₅ and R₄ may be the same or different and are hydrogen or alkylgroups containing from 1 to about 6 carbon atoms and A is an aromaticmoiety selected from the group consisting of (a) biphenyl, diphenylalkylidene having from 1 to about 6 carbon atoms in the alkylideneportion thereof, diphenyl sulfone, diphenyl sulfoxide, diphenyl ether,and diphenyl sulfide; (b) the diglycidyl derivatives of group (a); and(c) the diurethane derivatives of either group (a) or group (b).Additional examples include allyl acrylate, divinyl (trivinyl or higherhomologs) benzene, substituted divinyl benzenes, and analogouscompounds. Compounds such as bis-GMA and the urethane diacrylate formedby reacting hydroxyethyl acrylate, hydroxypropyl acrylate and theirmethacrylic homologs with 2,4,4-trimethylhexyl-1,6-diisocyanate areespecially useful, as are diallyl maleate, ethylene glycoldimethacrylate, trimethylolpropane trimethacrylate and thedimethacrylate ester of bisphenol-A.

Furthermore, mixtures of two or more crosslinking agents are useful inthe practice of the invention.

Preparation of Crosslinked Polymer Powder

Crosslinked polymer which may be prepared from the ingredients above orothers which are useful in the practice of a preferred interpenetratingnetwork embodiment of the invention are formed from monomers or blendsof monomers together with crosslinking agents. The monomers suitable foruse in the production of the crosslinked polymers include acrylic andlower alkyl acrylic acid esters, N-vinyl lactams, acrylamides,acrylonitriles, styrenes, alkenes, and urethanes. Preferredmonofunctional monomeric species useful in the preparation of thecomposition of the invention include acrylic and lower alkyl acrylicacid esters which generally conform to the general formula (IV):

wherein R₁ is hydrogen or an alkyl group including from 1 to about 6carbon atoms, and where R₂ is either (a) an alkyl or cycloalkyl groupincluding from 1 to about 20, and preferably from 1 to about 6 carbonatoms; (b) phenyl and (c) alkyl substituted phenyl in which the alkylgroups include form 1 to about 6 carbon atoms. Various substituents maybe present on either or both of the groups R₁ and R₂. Thus, hydroxyl,carboxyl, amino, thliol and halogen e.g., fluorine, chlorine, etc.)functionalities may be present, with the latter being preferred.Fluorine is an especially suitable and useful substituent.

The crosslinked polymer powders are produced by polymerizing a mixtureof the monomer or monomers and crosslinking agent or agents describedabove. The amount of crosslinking agent employed in the production ofthe crosslinked polymers used in the practice of the invention is acritical factor. It has been found that the capacity of particles ofpolymers so produced to swell with or to imbibe the liquid componentsforming the precursor blend of the invention, is directly related to theamount of crosslinking agent used in the production of such crosslinkedpolymers.

The physiochemical properties of the crosslinked polymer fillers usefulin the preferred interpenetrating network embodiment of the inventiondetermine the relative proportions of monomer and crosslinking agentused to formulate said suitable crosslinked polymers. Such crosslinkedpolymers must be sufficiently well crosslinked as to maintainsubstantially their structural identity when exposed to the liquidcomponents of the precursor blend of the invention. At the same time,they must not be so thoroughly crosslinked as to be incapable ofswelling with or imbibing such liquid components. Thus, it is convenientto describe the proportion of crosslinking agent by what it does ratherthan by what it is. In view of the fact that the crosslinked polymersare utilized in finely particulate form, as will be more fullyexplained, it is convenient to define the minimum amount of crosslinkingagent used therein as being that amount which is sufficient to cause theparticulate crosslinked polymer not to lose its particulate discretenessupon exposure to the liquid components of the invention. Similarly, themaximum amount of crosslinking agent used therein is that amount beyondwhich the resulting crosslinked polymer particles are unable to swellwith or further imbibe a significant portion of liquid components uponexposure thereto. In this regard, a quantity of crosslinked polymerparticles would be said to swell with or imbibe a significant portion ofliquid components if it swelled with or has imbibed at least 10% of itsown weight of such liquid. Preferably, an amount of crosslinking agentis used to provide a crosslinked polymer having the capacity to imbibefrom about 10 to about 500 percent of its own weight of liquidcomponents.

It will be clear to those skilled in the art that the minimum andmaximum values for the proportions of crosslinking agents suitable forinclusion in the crosslinked polymers of this invention will varydepending upon the chemical identity of the component monomers andcrosslinking agents. In general, however, the crosslinking agents maycomprise from as low as about 0.01% to as high as about 100 andpreferably from about 0.2% to about 40 by weight of the resultingcrosslinked polymer.

The production of the crosslinked polymer useful in the preferredinterpenetrating network embodiment of the invention from monomers andcrosslinking agents may be performed by any of the many processes knownto those skilled in the art. Thus, the polymers may be formed by heatinga mixture of the components to a temperature sufficient to causepolymerization, either with or without the addition of initiators. Forthis purpose, peroxy type initiators such as benzoyl peroxide, dicumylperoxide and other materials familiar to those skilled in the art may beemployed and the use of activators may be advantageous in someformulations. Alternatively, the crosslinked polymers of the inventionmay be formed from the constituents by photochemical or radiantinitiation utilizing light or high energy radiation.

The polymerization of the crosslinked polymers may be accomplished in awide variety of ways all of which are known to those skilled in the art.Thus, they may be formed by suspension polymerization (as taught by Grimin U.S. Pat. No. 2,673,194), emulsion polymerization, blockpolymerization. The crosslinked particles preferably have an averageparticle size should be from about 0.001 micron to about 500 microns. Itis preferred that at least 50% by weight of the particles have diametersbelow 150 microns and more preferably below 100 microns.

Uncrosslinked Polymer Powders

In addition to the crosslinked polymers described above, the polymercomponent of the precursor blend of a preferred embodiment of theinvention may comprise an uncrosslinked polymer. Such uncrosslinkedpolymer include those formed from any of the monofunctional monomerspecies which have been disclosed above as being useful for thepreparation of the crosslinked polymers used in the practice of theinvention. Thus, monomer species conforming to the formula above, theacrylic and C₁ to C₆ lower alkyl acrylic esters of aliphatic alcoholshaving from 1 to about 20 carbon atoms, or mixtures thereof, aresuitable as is vinylidene fluoride. Polymeric methyl methacrylate andcopolymers thereof are preferred. The uncrosslinked polymers may beformed from the monomers through any of the polymerization proceduresknown to those skilled in the art. Thus, thermal or photochemicalpolymerization, either with or without initiators, sensitizers,activators or chain transfer agents may be employed. Similarly, eitherbulk, suspension or emulsion polymerization may be utilized. Preferably,the uncrosslinked polymers should be characterized as having averagemolecular weight of from about 100,000 to about 2,000,000 g/mole, andespecially of from about 500,000 to about 900,000 g/mole. While thepolymers are used in particulate form, they differ from the crosslinkedpolymer filler in that, unlike the crosslinked polymers, theuncrosslinked polymers do not have a critical particle sizedistribution. Thus, polymer particles or beads of any conveniently smallsize such as about 50 microns, may be utilized. Smaller sizes arepreferred since they imbibe monomers and will dissolve therein morereadily, but larger sizes may be used as well.

Polymerizable Liquid Monomer Blend

Polymerizable monomers suitable for use in the formulation of theprecursor blend of a preferred embodiment of the invention may compriseany of a wide variety of monomers including those previously describedexamples provided in the preparation of crosslinked and uncrosslinkedpolymer powder. Thus, acrylic and lower alkyl acrylic acid esters,N-vinyl lactams, acrylimides, acrylamides, acrylonitriles, styrenes,alkenes, urethane acrylate or methacrylate and other monomeric speciesmay be employed in the practice of the invention.

Especially preferred examples of polymerizable monomers useful in thepractice a preferred embodiment of the invention include methyl-,ethyl-, isopropyl-, t-butyl-, octyl-, dodecyl-, cyclohexyl-,chloromethyl-, tetrachloroethyl-, perfluorooctyl-, hydroxyphenyl-,hydroxypropyl-, hydroxybutyl-, 3-hydroxyphenyl-, 4-hydroxyphenyl-,aminoethyl-, aminophenyl-, and thio-phenyl-, acrylate, methacrylate,ethacrylate, propacrylate, butacrylate and chloromethacrylate, as wellas the homologous mono-acrylic acid esters of bisphenol-A,dihydroxydiphenyl sulfone, dihydroxydiphenyl ether, dihydroxybiphenyl,dihydroxydiphenyl sulfoxide, and2,2-bis(4-hydroxy-2,3,5,6-tetrafluorophenyl)propane. Other suitablespecies will be apparent to those skilled in the art who will furtherrecognize that mixtures of two or more different polymerizable monomersmay be used. Polyfunctional Monomers And Oligomer Components OfPolymerizable Liquid Blend

Preferably, the crosslinking agents for the polymerizable monomerscomprise esters of unsaturated acids, e.g., acrylic, methacrylic,ethacrylic, propacrylic, butacrylic, etc. maleic, fumaric, citraconic,mesaconic, itaconic, malonic, or aconitic, etc., acids. Otherunsaturated acids will be readily apparent to those skilled in the art.These acids are preferably reacted with either unsaturated orpolyhydroxylic alcohols to form esters which are effectivepolyfunctional crosslinking agents for the monomeric species useful inthe practice of the invention. Thus, useful alcohols include allyl,methallyl, crotyl, vinyl, butenyl, isobutenyl and similar unsaturatedalcohols as well as polyols such as ethylene glycol, propylene glycol,butylene glycol, diethylene glycol, triethylene glycol, tetraethyleneglycol, pentaethylene glycol, glycerol, trimethylolpropane,pentaerythritol, dihydroxyphenol, alkylidene bisphenols such asbisphenol-A; 1,1-bis (4-hydroxyphenyl) methane; 4,4′-dihydroxybiphenyl;4,4′-dihydroxydiphenyl sulfone; dihydroxydiphenyl ether;dihydroxydiphenyl sulfoxide; resorcinol; hydroquinone; etc.

Preferred crosslinking agents used in the practice of the inventioninclude those previously described examples provided for in preparationof crosslinked and uncrosslinked polymer powders as well as, the estersof a monomeric dibasic unsaturated acid with an unsaturatedmonohydroxylic alcohol such as allyl acrylate, allyl methacrylate, vinylacrylate (methacrylate and homologs), dimethallyl fumarate, N-allylacrylamide, crotyl acrylate, allyl crotonate, allyl cinnamate, diallylmaleate, etc. Other preferred species are the di-, tri-, and higheresters of polyhydroxylic alcohols such as ethylene “glycol” diacrylate(dimethacrylate and C_(l)-C₆ homologs), trimethlolpropanetrimethacrylate, and the dimethacrylate ester of bis-phenol-A as well asother acrylate and allyl acrylate esters. In addition, the crosslinkingagent for the polymerizable monomers may be a glycidyl acrylate r allylacrylate, divinyl (trivinyl or higher homologs) benzene, substituteddivinyl benzenes, or analogous compounds. Furthermore, mixtures ofcrosslinking agents are useful in the practice of the invention.

Compounds such as those described herein above as crosslinking agentsand bis-G A and the urethane dimethacrylate formed from the reaction ofhydroxyethyl acrylate, hydroxypropyl acrylate and their methacrylatehomologs with 2,4,4-trimethylhexyl-1,6-diisocyanate (hereinafterreferred to as “urethane dimethacrylate” or “diacrylate”) are especiallyuseful, as are ethylene glycol dimethacrylate, trimethylolpropanetrimethacrylate and the dimethacrylate ester of bisphenol-A. Thecorresponding acrylates are similarly useful as is diallyl maleate.

Additional Ingredients

In addition to the components described above, (i.e., crosslinkedpolymer, uncrosslinked polymer, polymerizable monomer) the precursorblend may contain additional, optional, ingredients, such as,initiators, activators, pigments, fillers, radiopaquing agents, adhesionmodifiers, free radical or photochemical initiators. In this regard,peroxy type initiators such as dicumyl or benzoyl peroxide are useful.Similarly, pigments and fillers may be added to modify the appearance,density, and physical characteristics of the resultant dentalappliances. Inorganic materials, especially silica and titania,silicates and aluminates, glasses and ceramics are useful fillers andpigments, and a wide variety of other useful pigments and fillers willbe apparent to those skilled in the art. The fillers and radiopaquingagents may constitute a major part by weight of the compositions of theinvention. According to a preferred embodiment, the precursor blend ofthis invention may comprise admixtures of organic resin components andparticulate, inorganic filler in weight ratios of from about 1:2 toabout 2:1 or more depending on the specific gravity of inorganicingredients included.

Precursor Blends

The precursor blends in accordance with a preferred embodiment of theinvention are formulated by a mixing together of the constituent speciesin proper proportion, followed by aging or maturing. Several techniquesare available for this and others will be apparent to those skilled inthe art. Thus, it is possible to combine crosslinked polymer filler,self-lubricating particles, uncrosslinked polymer and polymerizableliquid bland in proper proportions including therewith, for example, aperoxide initiator and a pigment. This combination is then thoroughlymixed and aged to result in a precursor blend which has a uniformappearance. This blend may have the consistency of dough or may be moreor less mobile depending upon the desired use thereof. Particulateinorganic fillers or other modificants may be preferably added at thisstage in the formulation of the compositions if desired. Thecompositions thus formed may be alternatively molded, extruded, brushed,formed, worked or otherwise shaped in any conventional manner and causedto polymerize or cure to result in hard dental appliances havingsuperior properties. The application of heat or radiant energy isusually required for this polymerization or curing.

It is especially useful to mold the compositions of this invention intoartificial teeth for inclusion in prosthetic devices. It is to beunderstood, however, that the precursor blends are suitable for a verywide range of dental uses, including fillings, teeth, bridges, crowns,veneers, facings, denture base and denture reline materials, andorthodontic splint materials, and the like. The materials of theinvention may also be utilized for prosthetic replacement or repair ofvarious hard body structures such as bone and may be utilized forreconstructive purposes during surgery.

The nature of the chemical and physical relationships among thecomponents of the precursor blends of the invention is important to thepractice of the invention. Among these relationships is the necessitythat the crosslinked polymer particles be capable of swelling byimbibing the liquid components. Of similar importance is the requirementthat the uncrosslinked polymers, when included, be capable of dissolvingin the liquid components. The precursor blend formed by any of theuseful techniques described above is aged for a period of timesufficient to insure that in one embodiment the crosslinked polymer hasbecome substantially fully swollen with, interpenetrated by or hassubstantially imbibed the liquid crosslinking blend and that theuncrosslinked polymer, if used, has at least partially been dissolvedtherein. Thus, as used herein, “aged” or “aging” refer to themaintenance of the components of the precursor blend in association withone another in the blend for a period of time sufficient tosubstantially fully swell the crosslinked polymer particles with themixture of polymerizable monomer and crosslinking agent dissolvedtherein. Frequently, aging is manifested by a change in the consistencyof the mixture as equilibrium is approached. The time necessary toapproach such equilibrium will vary depending upon the blendingtechniques, the relative proportions of materials, the particle sizesand molecular weights of the polymers and the temperature extent in themixtures. In general, aging time of from one to seven days has beenfound to be adequate to approach the desired equilibrium. It is to beunderstood that it lies well within the abilities of those skilled inthe art to ascertain the optimum aging time for a formulation in view ofthe foregoing considerations. In accordance with this preferredtechnique, powder components including self-lubricating abrasionresistant particles are blended with a polymerizable liquid blend. Theprecursor blend is then aged for a period of time sufficient to permitthe crosslinked polymer particles to be substantially fully swollenwith, or interpenetrated by polymerizable blend. Precursor blends thusformed may be alternatively molded, brushed, extruded, formed, worked orotherwise shaped to form useful dental devices and articles. Othertechniques are presented in the examples which follow, and still otherswill be apparent to those skilled in the art.

Upon polymerization of the precursor blends in one embodiment a threedimensional structure is believed to be formed which may be denominatedas an interpenetrating polymeric network. The interpenetrating networkstructure which is believed to form is a major contributing factor tothe serendipitous combination of superior chemical and physiochemicalproperties which is exhibited by the articles constructed according tothe practice of the invention. Interpenetrating polymeric networks arerelated to, but distinct from, traditional graft polymers.

Interpenetrating Network Moldings

In accordance with a preferred embodiment of the invention aself-lubricating interpenetrating network may be viewed as beingcomposed of ultra high molecular weight polyethylene dispersed withintwo or more crosslinked, and hence three dimensionally arrayed,polymeric networks which do not necessarily have any covalent bonds incommon. While the two net-works may, indeed, be independent in the sensethat they need possess no covalent linkages between them; they arephysically trapped one “within” the other and cannot disassociate by anyphysical manipulation without the rupture of covalent bonds. Particulatecrosslinked polymer is allowed to swell with or imbibe monomer mixedwith crosslinking agent, and when the imbibed mixture of monomer andcrosslinking agent is subsequently caused to polymerize, aninterpenetrating polymeric network may be seen to be formed within theconfines of the particulate crosslinked polymer. It is believed that theaging process employed in the preparation of the precursor blends of theinvention is required to accomplish substantially full swelling withinterpenetration by or substantially complete inbibition of crosslinkingagent by the crosslinked polymer particles, and to approach anequilibrium thereof.

The American Dental Association specification number 15 specifies, “thestrength of the bond between tooth and resin is tested in tension. Theminimum bond strength is 30.9 MN/M²(4,480 psi; 315 Kg/cm²), which issufficient to prevent separation of the teeth from the resin denturebase in use.” This pertains to “acrylic denture base resin polymerizedby the heat processing technique.” The compositions of this inventionmeet or exceed this specification.

A unique, heterogeneous microstructure is exhibited by one embodiment,the preferred interpenetrating network embodiment of the invention. Oneexemplary method for observing this microstructure is as follows:

-   -   1. The tooth, or molded article, is sectioned and one section        patted in epoxy against a flat surface.    -   2. The sectioned surface of the potted specimen is polished to a        smooth surface using nos. 320, 400 and 600 grit silicon carbide        papers wet continuously with water.    -   3. A final polish is obtained using an aqueous slurry of 0.3        micron Al₂O₃ on a chamais.    -   4. The polished surface of the section is exposed for four        minutes to the vapors of boiling concentrated nitric acid; the        microstructure is oxidatively exposed by this etching procedure        and is best captured by photomicrography at 260× magnification.

The microstructure thus observed is heterogeneous and comprises what maybest be described as particles suspended in a matrix. The particles arebelieved to be identifiable with the particulate crosslinked polymers ofthe precursor blend which have been swollen by and inter-penetrated withthe monomer and crosslinking agent. By comparison with conventionalcomposite compositions containing only rigid inorganic fillers, thearticles formed according to the present invention exhibit amicrostructure in which the structure is much more closely packed. It isto be understood that this methodology, while of wide application in theexamination of the microstructure of the novel compositions of theinvention, is not exclusive. Other techniques involving greater orlesser magnification and other means of visualization are also useful indisclosing the structure. Distributed throughout this structure isparticulate self-lubricating particles.

Preferably teeth formed in accordance with a preferred embodiment of theinvention are from 0.1 to 80 percent by weight self-lubricating materialhaving a kinetic coefficient of friction less than 0.25 when measuredusing ASTM D 1894-78 and 5 psi.

Preferably, the self-lubricating materials for use in accordance withthe invention have an average largest dimension (or diameter) from 0.5to 80 microns. More preferably, such particles have an average largestdimension (or diameter) less than 70 microns. Most preferably, theself-lubricating particles for use in accordance with the invention havean average largest dimension (or diameter) from 1 to 80 microns. Mostpreferably, the self-lubricating particles for use in accordance withthe invention have an average largest dimension (or diameter) from 2 to20 microns. A preferred embodiment of the invention provides a toothincluding polyethylene having a molecular weight of at least 500,000 andpreferably a molecular weight of at least 1,000,000, and a particle sizeless than 80 microns. In a preferred embodiment of the invention adental prosthesis is provided including polyethylene, particles having aparticle size less than 80 microns and a molecular weight of at least3,000,000 functionalized by gas plasma treatment so as to be bondable tothe matrix polymer.

The following examples describe certain representative embodiments ofthis invention and will serve further to illustrate the nature thereof.It is to be understood that the examples are merely illustrative, and donot in any way limit the scope of the invention as defined by theclaims. All percentages are by weight.

COMPARATIVE EXAMPLE 1

Prior Art IPN Tooth

A precursor blend is prepared from the following composition:

37.81% methyl methacrylate 0.23% benzoyl peroxide 7.96%2,2-bis(4-methacryloxyphenyl)propane 35.62% poly(methylmethacrylate-co-ethylene dimethacrylate) (99.2:0.8) 17.80% poly(methylmethacrylate) 0.58% pigment 100.00%

The crosslinked polymer is in the form of particles, 46% by weight ofwhich were below 74 microns in size, the balance being below about 500microns in size. The poly-(methyl methacrylate) have an averagemolecular weight of 800,000 g/mole.

The benzoyl peroxide and 2,2-bis(4-methacryloxy-phenyl)propane aredissolved in the methyl methacrylate at ambient temperature to form amonomer solution. The polymers and pigment are charged to a planetarydough mixer containing the monomer solution and the charge is stirreduntil visibly homogeneous. Prosthetic teeth are molded from theresultant precursor blend mixture after it is aged at ambienttemperature for seven days. The resulting teeth grind with a dusty, finedebris, bond to denture base and are impact and wear resistant.

EXAMPLE 2

Interpenetrating Network Teeth

The method described in Example 1 is used to prepare a precursor blendfrom which prosthetic teeth are molded having the following composition:

26.83% methyl methacrylate 4.00% ultrahigh molecular weight polyethylene(Hoechst, AG) treated with oxygen and fluorine (50 micron largestparticle dimension) .17% benzoyl peroxide 2.11% 2,2,2-trifluoroethylacrylate 2.37% ethylene glycol dimethacrylate 1.52% reaction product ofhydroxypropyl methacrylate with 2,2,4-trimethylhexyl-1,6-disocyanate(2:1) (urethane dimeth- acrylate) (UDMA) 41.30%poly(methyl methacrylate-co-ethylene dimethacrylate) (98.8:1.2) 20.65%poly(methyl methacrylate) 1.05% pigment 100.00%

A suitable gel-like consistency for molding prosthetic teeth is obtainedafter aging at ambient temperature for 24 hours. The high molecularweight self-lubricating polyethylene had a molecular weight ofapproximately 4,000,000 and was obtained from Hoechst-Celanese Inc.Particle surfaces were oxyfluorinated in the presence of a fluorine andoxygen at high temperature to be capable of bonding to the matrixmaterial within which it was dispersed.

EXAMPLE 3

Interpenetrating Network Teeth

The following composition yielded a precursor blend which is molded intoprosthetic teeth after processing according to the technique of Example1.

45.83% methyl methacrylate 4.00% ultrahigh molecular weight polyethylenetreated with oxygen and fluorine (50 micron largest particle dimension0.17% benzoyl peroxide 11.50% bis-GMA 24.80% poly(methylmethacrylate-co-ethylene dimethacrylate) (70:30) 11.90% poly(methylmethacrylate) 1.80% pigment 100.00%

EXAMPLE 4

Interpenetrating Network Teeth

A two-step “preswell” mixing method is used to prepare a precursor blendfrom which prosthetic teeth are molded. The blend has the followingcomposition:

Step 1 40.40% methyl methacrylate 4.00% ultrahigh molecular weightpolyethylene treated with oxygen and fluorine (50 micron largestparticle dimension) 0.25% benzoyl peroxide 6.00% reaction product ofhydroxypropyl methacrylate with 2,2,4-trimethylhexyl- 1,6-disocyanate(2:1) (urethane dimeth- acrylate) (UDMA) 1.50%2,2-bis(4-methacryloxyphenyl)propane 47.85% poly(methylmethacrylate-co-ethylene dimethacrylate) (90:10) 100.00%The crosslinked polymer is in the form of particles, 50% by weight ofwhich are below 100 microns in size, the balance being below about 500microns in size.

Step 2 28.14% poly(methyl methacrylate) 60.43% methyl methacrylate 0.36%benzoyl peroxide 10.20% 2,2-bis(4-methacryloxyphenyl)propane 0.87%pigment 100.00%The poly(methyl methacrylate) has an average molecular weight of 850,000g/mole.

The weight ratio of Step 1 to Step 2 material in this example is 1.14 to1.00. Step 1 is achieved by preparing a solution of the monomers,crosslinkers and initiator and adding the crosslinked copolymer. Thismixture is stirred for about two minutes to wet the polymer, cappedagainst monomer loss, and held for one week at about 10° C. temperature.The crosslinked copolymer completely absorbed the monomer solutionduring the one week “pre-swell” period. Although the copolymer isswollen by this process, the integrity of the individual copolymerparticles is maintained. This “preswell” mixture is not gel-like, buthas the consistency of a rubbery, spongy mass which is easily crumbled.

Step 2 is achieved by charging “preswell”, obtained in Step 1, to aplanetary dough mixer and mixing sufficiently so as to break the“preswell” mass of own to a fine consistency. The poly(methylmethacrylate) and pigment are added to the mixer and mixing is continueduntil a homogeneous dispersion is obtained. The solution of monomer andinitiator, cited in the Step 2 composition, is charged to the mixer;mixing continued until a homogeneous, gel consistency is obtained. Thegel-like mix is transferred to a holding container and aged at about 10°C. temperature until a suitable consistency for molding prosthetic teethis obtained, approximately three days.

EXAMPLE 5

One-Component Filled Radiation Curable Material

The following gel composition, containing an inorganic filler andprepared by the method described in Example 1, is polymerized by visiblelight radiation using a Caulk MAX photo-cure lamp (registered trademarkof Dentsply International):

21.21% methyl methacrylate 3.00% ultrahigh molecular weight polyethylenetreated with oxygen and fluorine (50 micron largest particle dimension)2.96% butyl methacrylate 0.27% camphoroquinone 0.43% dimethylaminobenzoic acid ethyl ester 2.08%2,2-bis(4-methacryloxyethoxyphenyl)-propane 1.13% tetraethylene “glycol”dimethacrylate 1.13% neopentyl “glycol” dimethacrylate 19.45%poly(methyl methacrylate-co-2,2-bis- (4-methacryloxyphenyl)propane)(99.8:0.2) 12.23% poly(methyl methacrylate) 35.66% silane treated, fine(12 micron) particle quartz 0.45% pigment 100.00%

EXAMPLE 6

Dental Veneers

A two step preswell process is used to mix a one part dental veneermaterial. A blend is prepared from the following:

Step 1 2.99% methyl methacrylate 3.00% ultrahigh molecular weightpolyethylene treated with oxygen and fluorine (50 micron largestparticle dimension) 0.51% benzoyl peroxide 45.26% reaction product ofhydroxypropyl methacrylate with 2,2,4-trimethylhexyl- 1,6-disocyanate(2:1) (urethane dimeth- acrylate) (UDMA) 48.24% poly(methylmethacrylate-co-ethylene dimethacrylate) (99.8:0.2) 100.00%

The benzoyl peroxide is dissolved in the methyl methacrylate and blendedwith the urethane dimethacrylate. This solution is then mixed with thepoly(methyl methacrylate-co-ethylene dimethacrylate) (99.8:0.2) andUHMWPE. The mixture is stored in the dark in a sealed jar to become the“pre-swell” blend. The crosslinked polymer i.e. poly(methylmethacrylate-co-ethylenedimethacrylate) (99.8:0.2) is in the form offine particles at least 50% by weight are below 100 microns in size, andthe balance below 500 microns in size. After one month storage the fullyswollen crosslinked polymer “preswell” blend is admixed as follows:

Step 2 48.84% “preswell” blend from Step 1 51.03% silane treatedmicrofine silica 0.13% acrylic acid 100.00%These components are mixed on a three roll mill with minor amounts ofpigments as required until a uniformly shaded paste is obtained.

A veneer is prepared on an opaqued crown by the well known build upmethod. A dentin shade veneer paste is built up on the crown by hand andinstrument modelling. Next, an incisor shade veneer paste is built ontop of the dentin. The veneer is polymerized by immersion in a 90° C.water bath under three bars air pressure. Veneers are also polymerizedby immersion in glycerin in a similar manner. The finished veneer has ahigh gloss and good aesthetic appearance. The veneer has three times thewear resistance of conventional acrylic veneers by a prophy abrasiontest. The veneer can be readily shaped by grinding, yielding a dustydebris, and then is readily polished to a smooth, high gloss finish. Theveneer is resistant to chemicals and stains, has good impact strengthand is repairable. The veneer paste is stable at ambient conditions. Theveneer paste is stable for nine months at ambient and seventy days at50° C.

EXAMPLE 7

Dental Veneer

A two step preswell process is used to mix a one part dental veneer fromthe following composition:

Step 1

“Preswell” blend from Example 6 aged 22 days at ambient.

Step 2 57.29% “preswell” blend from Step 1 41.67% silane treatedmicrofine silica 1.04% acrylic acid 100.00%The components are mixed on a three roll mill until visually uniform.The putty-like material is formed into a dental veneer and polymerizedby immersion in 90° C. glycerin at three bars pressure for ten minutesto yield shapable, durable and aesthetically superior appliances.

EXAMPLE 8

TOOTH MATERIAL 60.00% polymethymethacrylate 800,000 average molecularweight 31.35% methylmethacrylate 3.48% ethylene glycol dimethacrylate0.17% benzoyl peroxide 5.0% ultra high molecular weight polyethylene,surface treated 100.00%Ultra high molecular weight polyethylene (PRIMAX) and polymethylmethacrylate 800,000 average molecular weight are dispersed in oneanother in a high shear mixer. The powder and liquid components are thencombined in a planetary mixer and stored for one week to achieve asuitable consistency for molding. Teeth are molded using heat andpressure in closed mold to form artificial teeth according to theinvention.

EXAMPLE 9

Tooth Material

A precursor blend is prepared from the following composition:

37.3% silica, as in EXAMPLE 8, but not silanated 17.4% methylmethacrylate 0.5% benzoyl peroxide 17.4% ethylene glycol dimethacrylate14.9% poly(methyl methacrylate) 25.% ultrahigh molecular weightpolyethylene as in EXAMPLE 8. 100.00%Ingredients are mechanically mixed in a closed container until a viscouspaste is obtained. This paste is introduced into a tooth mold andpolymerized for 4 minutes at 110° C. The artificial tooth thus obtainedshows clear opalescence, i.e. it appears yellowish in transmitted lightand of a blue-white transparency in incident light. The ball indentationhardness is 2800 kg/cm² as compared with 1400 kg/cm² in the case ofcomparison teeth compared on basis of the customary methacrylate.

EXAMPLE 10

Self-Lubricating Abrasion Resistant IPN Tooth

A precursor blend is prepared from the following composition:

18.16% high molecular weight poly(methyl meth- acrylate) polymer 36.26%poly(methyl methacrylate-co-ethylene dimethacrylate) (99.2:0.8) 7.45%2,2-bis(4-methacryloxyphenyl)propane 6.82% reaction product ofhydroxypropyl methacrylate with 2,2,4-trimethylhexyl- 1,6-disccyanate(2:1) (urethane dimeth- crylate) (UDMA) 28.52% methylmethacrylate 0.21%benzoyl peroxide 2.28% ultra high molecular weight polyethylene 0.33%pigments 100.00%

The UHMWPE and crosslinked polymer is in the form of particles, 46% byweight of which were below 74 microns in size, the balance being belowabout 500 microns in size. The poly-(methyl methacrylate) has an averagemolecular weight of 800,000 g/mole.

The tooth is evaluated for wear resistance in a computer programmedchewing device (Journal of Prosthetic Dentistry, Volume 54, Number 2,August 1985, pages 273-280). It demonstrates a volume loss from wearagainst a natural tooth of 0.021 mm³, compared to 0.032 mm³ loss forcommercial tooth, BIOFORM (Dentsply) prepared in general according toU.S. Pat. No. 4,698,373.

TABLE 1 Comparison of tooth wear: Tooth wear Percent Reduction Exampleg/10,000 cycles in wear  1 (Prior Art 0.0327 0   tooth) 10 0.0247 25

It should be understood that while the present invention has beendescribed in considerable detail with respect to certain specificembodiments thereof, it should not be considered limited to suchembodiments but may be used in other ways without departure from thespirit of the invention and the scope of the appended claims.

1. A method of making an interpenetrating polymer network artificialdental tooth having a reduced coefficient of friction and adapted foruse in combination with denture base formed into a denture, comprising:forming a tooth body consisting essentially of an interpenetratingpolymer network having a polymer matrix, said interpenetrating polymernetwork being formed by polymerization of monomers of a polymer matrixforming composition, said polymer matrix forming composition comprisinga blend of particulate material and liquid polymerizable monomers, saidliquid polymerizable monomers comprising monofunctional polymerizablemonomer and di or polyfuncational crosslinking agent reactive with saidpolymerizable monomer, said particulate material comprising from about10% to about 70% by weight of said tooth body, said particulate materialcomprising self lubricating polymer particles and crosslinked polymerparticles having average diameter up to about 500 microns, saidcrosslinked polymer particles being sufficiently crosslinked as tomaintain substantially their structural identity when exposed to saidmonomers, said crosslinked polymer particles being substantially swollenby imbibing at least 10% by weight of said crosslinked polymer particlesof said monomers, said self-lubricating particles comprising a polymerof the general formula:

wherein p is an integer from 100 to 1,000,000, (--) is a single or adouble bond; when (--) is a double bond R, R′ and R₁₁ independently arehydrogen, or a lower alkyl of from 1 to 6 carbons, and R₉ and R₁₀ arenot present; when (--) is a single bond R, R′, R₉, R₁₀, and R₁₁independently are hydrogen, fluorine or a lower alkyl of from 1 to 6carbons, said self-lubricating particles being effectively bonded to anddistributed in said polymeric matrix, said interpenetrating polymernetwork artificial tooth comprising at least 0.1 percent by weight ofsaid self-lubricating particles and said interpenetrating polymernetwork artificial tooth effectively having a reduced coefficient offriction compared to said interpenetrating polymer network artificialtooth from which said self-lubricating particles are omitted, said toothbeing adapted to bond with a bond strength between said tooth and saiddenture base of at least 4,480 psi.
 2. The method of claim 1 whereinsaid self-lubricating particles have a particle size less than 500microns.
 3. The method of claim 1 wherein about 46 percent by weight ofsaid self-lubricating particles have a particle size less than 74microns.
 4. An improved method of making an interpenetrating polymernetwork artificial dental tooth adapted for use in combination withdenture base formed into a denture, comprising: forming a tooth having atooth body consisting essentially of an interpenetrating polymer networkhaving a polymer matrix, said interpenetrating polymer network beingformed by polymerization of monomers of a polymer matrix formingcomposition, said polymer matrix forming composition comprising a blendof particulate material and liquid polymerizable monomers, said liquidpolymerizable monomers comprising monofunctional polymerizable monomerand di or polyfuncational crosslinking agent reactive with saidpolymerizable monomer, said particulate material comprising from about10% to about 70% by weight of said tooth body, said particulate materialcomprising crosslinked polymer particles having average diameter up toabout 500 microns, said crosslinked polymer particles being sufficientlycrosslinked as to maintain substantially their structural identity whenexposed to said monomers, said crosslinked polymer particles beingsubstantially swollen by imbibing at least 10% by weight of saidcrosslinked polymer particles of said monomers, the improvement whereinsaid particulate material comprises self-lubricating particleseffectively bonded to and distributed in said polymeric matrix, saidself-lubricating particles comprising a polymer of the general formula:

wherein p is an integer from 100 to 1,000,000, (- -) is a single or adouble bond; when (- -) is a double bond R, R′ and R₁₁ independently arehydrogen, or a lower alkyl of from 1 to 6 carbons, and R₉ and R₁₀ arenot present; when (- -) is a single bond R, R′, R₉, R₁₀, and R₁₁independently are hydrogen, fluorine or a lower allyl of from 1 to 6carbons said tooth comprising at least 0.1 percent by weight of saidself-lubricating particles, said tooth being adapted to bond with a bondstrength between said tooth and said denture base of at least 4,480 psi.5. The method of claim 4 wherein said self-lubricating particles have aparticle size less than 500 microns.
 6. The method of claim 4 whereinabout 46 percent by weight of said self-lubricating particles have aparticle size less than 74 microns.
 7. An improved method of making aninterpenetrating polymer network artificial dental tooth adapted for usein combination with denture base formed into a denture, comprising:forming a tooth having a tooth body consisting essentially of aninterpenetrating polymer network having a polymer matrix, saidinterpenetrating polymer network being formed by polymerization ofmonomers of a polymer matrix forming composition, said polymer matrixforming composition comprising a blend of particulate material andliquid polymerizable monomers, said liquid polymerizable monomerscomprising monofunctional polymerizable monomer and di orpolyfuncational crosslinking agent reactive with said polymerizablemonomer, said particulate material comprising from about 10% to about70% by weight of said tooth body, said particulate material comprisingcrosslinked polymer particles having average diameter up to about 500microns, said crosslinked polymer particles being sufficientlycrosslinked as to maintain substantially their structural identity whenexposed to said monomers, said crosslinked polymer particles beingsubstantially swollen by imbibing at least 10% by weight of saidcrosslinked polymer particles of said monomers, said product being madeby the process comprising adding self-lubricating particles to saidparticulate material, said self-lubricating particles being effectivelybonded to and distributed in said polymeric matrix, saidself-lubricating particles comprising a polymer of the general formula:

wherein p is an integer from 100 to 1,000,000, (- -) is a single or adouble bond; when (- -) is a double bond R, R′ and R₁₁ independently arehydrogen, or a lower alkyl of from 1 to 6 carbons, and R₉ and R₁₀ arenot present; when (- -) is a single bond R, R₁, R₉, R₁₀, and R₁₁independently are hydrogen, fluorine or a lower alkyl of from 1 to 6carbons, said tooth comprising an effective amount of saidself-lubricating particles, said tooth being adapted to bond with a bondstrength between said tooth and said denture base of at least 4,480 psi.8. The method of claim 7 wherein said self-lubricating particles have aparticle size less than 500 microns.
 9. The method of claim 7 whereinabout 46 percent by weight of said self-lubricating particles have aparticle size less than 74 microns.
 10. A method of making aninterpenetrating polymer network artificial dental tooth having areduced coefficient of friction and adapted for use in combination withdenture base formed into a denture, comprising: forming a tooth bodycomprising an interpenetrating polymer network having a polymer matrix,said interpenetrating polymer network being formed by polymerization ofmonomers of a polymer matrix forming composition, said polymer matrixforming composition comprising a blend of particulate material andliquid polymerizable monomers, said particulate material comprising selflubricating polymer particles having a particle size less than 500microns and crosslinked polymer particles having average diameter up toabout 500 microns, said self-lubricating particles being effectivelybonded to and distributed in said polymeric matrix, saidself-lubricating particles comprising a polymer of the general formula:

wherein p is an integer from 100 to 1,000,000, (- -) is a single ordouble bond; when (- -) is a double bond R, R′ and R₁₁ independently arehydrogen, or a lower alkyl of from 1 to 6 carbons, and R₉ and R₁₀ arenot present; when (- -) is a single bond R, R′, R₉, R₁₀, and R₁₁independently are hydrogen, fluorine or lower alkyl of from 1 to 6carbons, said interpenetrating polymer network artificial toothcomprising at least 0.1 percent by weight of said self-lubricatingparticles and said interpenetrating polymer network artificial tootheffectively having a reduced coefficient of friction compared to saidinterpenetrating polymer network artificial tooth from which saidself-lubricating particles are omitted, said tooth being adapted to bondwith a bond strength between said tooth and said denture base of atleast 4,480 psi.